Why would mean pulmonary pressure be 5x less than mean systemic pressure?

Not a fixed percent: You can't say that the systemic arterial pressure is always 5 times the pulmonary arterial pressure because each can vary. But the branching of the pulmonary arteries tends to lower resistance of blood flow and keep the pulmonary arterial pressures lower.

Blood flows: From high pressure areas to low pressure areas. On the venous side of the circulation, the lungs are the last stop and have to have a lower pressure. Now on the arterial side, the blood has to be pumped with enough force to circulate throughout the body. As dr white noted, the ratio is not fixed.